The photovoltaic (PV) module is the basic building block of PV electrical systems. A PV module is composed of interconnected cells that are encapsulated between a glass cover and weatherproof backing. The modules are typically framed in aluminum frames suitable for mounting. The term “solar panel” is often used to refer to a PV module. However, the same expression is also used in reference to solar water heating systems, so to avoid confusion, “photovoltaic module” is preferred.
The factors that affect the output of a solar power system should be understood so that a user has realistic expectations of overall system output and economic benefits under variable weather conditions over time. The amount of useful electricity generated by a PV module is directly generated to the intensity of light energy, which falls onto the conversion area. So, the greater the available solar resource, the greater is the electricity generation potential. The tropics, for instance, offer a better resource for generating electricity than is available at high latitudes. It also follows that a PV system will not generate electricity at night, and it is important that modules are not shaded. If electricity is required outside daylight hours, or if extended periods of bad weather are anticipated, some form of storage system is essential.
Material
Among other things, the performance of a PV module depends on the cell material. The conversion efficiency of amorphous silicon modules varies from 6 to 8%. Modules of multi-crystalline silicon cells have a conversion efficiency of about 15%. Mono-crystalline silicon modules are the most efficient; their conversion efficiency is about 16%. Typical sizes of modules are 0.5×1.0 m2 and 0.33×1.33 m2, made up of about 36 PV cells.
Standard Test Conditions
Solar modules produce DC electricity. The DC output of solar modules is rated by manufacturers under Standard Test Conditions (STC). These conditions are easily recreated in a factory, and allow for consistent comparisons of products, but need to be modified to estimate output under common outdoor operating conditions. STC conditions are: solar cell temperature=25° C.; solar irradiance (intensity)=1000 W/m2 (often referred to as peak sunlight intensity, comparable to clear summer noon time intensity); and solar spectrum as filtered by passing through 1.5 thickness of atmosphere (ASTM Standard Spectrum). A manufacturer may rate a particular solar module output at 100 Watts of power under STC, and call the product a “100-watt solar module.” This module will often have a production tolerance of +/−5% of the rating, which means that the module can produce 95 Watts and still be called a “100-watt module.” To be conservative, it is best to use the low end of the power output spectrum as a starting point (95 Watts for a 100-watt module). FIG. 1 is a graphical presentation of the current versus the voltage (I-V curve) from a photovoltaic cell as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance; this can be called “factory performance” or performance of a PV cell under ideal conditions.
Spectrum
The electrical current generated by photovoltaic devices is also influenced by the spectral distribution (spectrum) of sunlight. It is also commonly understood that the spectral distribution of sunlight varies during the day, being “redder” at sunrise and sunset and “bluer” at noon. The magnitude of the influence that the changing spectrum has on performance can vary significantly, depending on the photovoltaic technology being considered. In any case, spectral variation introduces a systematic influence on performance that is time-of-day dependent. Similarly, the optical characteristics of photovoltaic modules or pyranometer can result in a systematic influence on their performance related to the solar angle-of-incidence.
Temperature
Module output power reduces as module temperature increases. When operating on a roof, a solar module will heat up substantially, reaching inner temperatures of 50 to 75 C degrees. For crystalline modules, a typical temperature reduction factor recommended by the CEC is 89% or 0.89. Therefore, the “100-watt” module will typically operate at about 85 Watts (95 Watts×0.89=85 Watts) in the middle of a spring or fall day, under full sunlight conditions. To ensure that PV modules do not overheat, it is essential that they be mounted in such a way as to allow air to move freely around them. This is a particularly important consideration in locations that are prone to extremely hot midday temperatures. The ideal PV generating conditions are cold, bright, sunny days.
Dirt and Dust
Dirt and dust can accumulate on the solar module surface, blocking some of the sunlight and reducing output. Much of California has a rainy season and a dry season. Although typical dirt and dust is cleaned off during every rainy season, it is more realistic to estimate system output taking into account the reduction due to dust buildup in the dry season. A typical annual dust reduction factor to use is 93% or 0.93. Therefore, the “100-watt module,” operating with some accumulated dust may operate, on average, at about 79 Watts (85 Watts×0.93=79 Watts).
Mismatch and Wiring Losses
The maximum power output of the total PV array is always less than the sum of the maximum output of the individual modules. This difference is a result of slight inconsistencies in performance from one module to the next, and is called “module mismatch” and can amount to at least a 2% loss in system power. Power is also lost to resistance in the system wiring. These losses should be kept to a minimum but it is often difficult to keep these losses below 3% for the system. A reasonable reduction factor for these losses is 95% or 0.95.
DC to AC Conversion Losses
The DC power generated by the solar module must be converted into common household AC power using an inverter. Some power is lost in the conversion process, and there are additional losses in the wires from the rooftop array down to the inverter and out to the house panel. Modern inverters commonly used in residential PV power systems have peak efficiencies of 92% to 94% indicated by their manufacturers, but these again are measured under well-controlled factory conditions. Actual field conditions usually result in overall DC-to-AC conversion efficiencies of about 88% to 92%, with 90% or 0.90 a reasonable compromise. So the “100-watt module” output, reduced by production tolerance, heat, dust, wiring, AC conversion, and other losses should translate into about 68 Watts of AC power delivered to the house panel during the middle of a clear day (100 Watts×0.95×0.89×0.93×0.95×0.90=67 Watts).